In recent years, transparent sheets and boards resistant against around 80° C., and moulds thereof, such as blister packs, automatic vendor windows, containers for foods and beverages (resistant against around 80-90° C.), containers for heating in microwave ovens (resistant against around 110-14° C.), lunch boxes, and containers for heating in oven ranges (resistant against around 180-220° C.), are being needed more and more as quality of life (QOL) more highly develops. Conventionally, as candidates of resins of these containers, polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET, non-stretched A-PET or white crystallised C-PET), polybutylene terephthalate (PBT), and polycarbonate (PC) have been proposed and some of them have been used. These resins have both advantages and disadvantages with respect to quality, including heat-resistance, processability, transparency, and strength etc.; costs; environmental adaptability; and safety etc.
For instance, polypropylene (PP) is inexpensive and easy to be processed. However, PP is not so good at transparency and shows elusion of a stabiliser etc. contained in PP at around 70° C. or higher. Polystyrene (PS) is inexpensive and shows good processability and transparency. However, PS has a safety problem with respect to residual monomers, and other problems with respect to weakness of resulting containers and deformation of resulting lunch boxes at around 110° C. or higher, which is the heat-resistance temperature for microwave ovens.
In general, sheets and containers thereof made of non-stretched amorphous PET (A-PET) have excellent transparency, strength, and environmental adaptability. However, their glass transition temperature (Tg) is low around 65-75° C., and they gradually deform at 80° C. or higher. Moreover, at around 110° C. or higher, which is the heat-resistant temperature for microwave ovens, A-PET has disadvantages of whitening by crystallisation, embrittlement, and deformation. Transparent films of biaxially oriented PET have heat-resistance against around 180°° C. However, they have high crystallisation degree (around 40%) due to molecular orientation by stretch direction. They are difficult to be pressure-formed into mould articles. Biaxially oriented sheets and boards are difficult to be obtained and are expensive.
A crystallised white PET (Crystalline-PET, C-PET) is excellent in heat-resistance against oven ranges, and environmental adaptability. However, resulting final mould products are difficult to be manufactured and expensive because a polyethylene medium with inorganic crystallisation seeds such as mica must be blended with PET and the resulting mixture is formed into white C-PET sheets (see e.g. Japanese Patent Publication No. 64-9179) and then moulded by two stage of vacuum-pressure-forming in a double-mould process for a long processing time (see e.g. Japanese Patent Publication No. 6-26854).
Furthermore, 140° C.-resistant sheets and containers have been successfully manufactured using a composition composed of 70-100 parts by weight of PBT resins and 30-0 parts by weight of PET resins (see e.g. Japanese Patent No. 2553228). However, use of PBT more than PET makes final mould products more expensive and brings a disadvantage that homogeneous moulds are difficult to be obtained due to the composition which whitens a part of the moulds.
Still furthermore, polycarbonate (PC) is excellent in transparency, strength, and heat-resistance but is expensive. Hence final products such as sheets etc. also become expensive and thus cannot be responsible to their demands. Still furthermore yet, Bisphenol-A, the main starting material of PC, has a problem in environmental adaptability, such as environmental hormones, and thus cannot be responsible to the demands.
In recent years, necessity for recycling plastic products used and recovered from production processes in plants or from general consumers has been globally recognized from a viewpoint of saving resources and preserving environment. In particular, recovery and recycle of large amounts of used PET bottles, films, sheets, and the like are actively being progressed. It has become possible to obtain them at prices only half of those of commodity resins. However, molecular weights of used and recovered PET have already decreased, compared to those of new virgin PET. For example, molecular weights of flakes of large amounts of recovered PET bottles have been already almost half reduced. Therefore, reuse of recovered PET only as a base resin results in poor mouldability. Resulting moulded products are weak and poor at impact-strength, hence quality as good as that of virgin PET bottles cannot be ensured. As a result, there are provided only fibres which can be moulded despite their low molecular weights, sheets of poor quality, and the like. Thus, their applications are limited within a narrow field. Furthermore, transparent PET polyesters have a Tg around 70° C. Hence they are glassy at room temperature (RT) and thus poor at cold brittleness, that is cold-resistance, and impact-strength. Since they do not have heat-resistance against 80° C. or higher, they are difficult to be developed in new applications.
On the other hand, polyolefins such as polyethylene, polypropylene etc. are used for films, sheets, containers, and so on in large quantities. However, it is known that such polyolefins are far inferior, in transparency, stiffness and hardness, to rigid vinyl chloride, PET polyesters, and polystyrene.
One method for solving these problems is a method for recovering and increasing molecular weights. With respect to PET polyesters, there are known methods for recovering molecular weights by solid phase polymerisation, for increasing molecular weights by reaction of chain-extender (coupling agent) with polyester terminal groups, and for adding other resins such as elastomers to enhance mechanical properties.
As the chain-extenders, coupling agents, there is proposed use of compounds having coupling hands or functional groups such as isocyanate, oxazoline, epoxide, aziridine, carbodiimide etc. However, there are many restrictions in terms of reactivity, heat-resistance, and stability etc., hence there are few useful chain-extenders. Amongst them, epoxy compounds are relatively useful. For example, monoepoxy compounds (Japanese Patent Laid-open No. 57-161124 A) and diepoxy compounds (Japanese Patent Laid-open No. 07-166419 A, Japanese Patent Publication No. 48-25074 B, Japanese Patent Publication 60-35944 B, etc.). However, they have many problems with respect to reaction speed, gel generation, melt viscosity, solubility, heat-stability, physical properties of resulting moulded products, and so on, and thus are difficult to be put to practical use.
On the other hand, there is proposed a method of increasing molecular weights of recovered PET polyesters by melting and mixing with a bifunctional epoxy resins and sterically hindered hydroxyphenylalkyl phosphonates (esters) (Japanese Patent Laid-open No. 08-508776 A (PCT)). In this method, the reaction rate is relatively rapid. However, the sterically hindered hydroxyphenylalkyl phosphonates (esters) are expensive and toxic, hence there are still remaining problems in practical use in industries where low costs for recovery and recycle are required and in food packaging where safety is required.
Furthermore, a method previously proposed by the inventors of the present invention (U.S. Pat. No. 6,506,852 B2), in which moderate molecular weight PET polyesters such as recovered PET bottles and virgin PET after condensation reaction were melted, mixed and reacted with both bifunctional epoxy compounds and polyfunctional epoxy compounds under specific catalysts, increased the molecular weights and the melting tension to allow to mould films and sheets but not to allow to obtain sufficient acknowledgements of heat-resistance. The inventors have further enabled also to mould foaming sheets (PCT WO 00/20491). However, moulded products from the foaming sheets are slightly weak and improvement of impact-strength and cold-brittleness, cold-resistance, are required. For instance, as packaging materials for frozen foods, moulded products not broken even at a low temperature of −20 to −30° C., while heat-resistant against 130° C. preferably, 230° C. more preferably, are needed.
Transparent sheets, boards, and containers of heat-resistant against around 80-130° C. have an advantage that their insides can be seen, and thus have big markets of, for example, blister packs, automatic vendor windows, containers for heated foods and for heating in microwave ovens, lunch boxes etc. Transparent blister packs as containers for small domestic electric products and OA instruments must have heat-resistance against 80° C., which is the maximum temperature at ship bottoms upon export or import. Transparent windows of automatic vendors must have heat-resistance against 80-90° C. derived from their display lights, hot cans to be sold etc.
On the other hand, containers highly heat-resistant against 180-230° C. are mainly used for oven ranges but require heat-resistance against temperatures around 100° C. higher than those of usual, not highly, transparent heat-resistant containers. Due to these much higher temperatures, usual moulding conditions give no transparency and give white opaque containers because of diffuse reflection, which is because sizes of spherical crystals of the resins are greater than wave lengths of visual lights. The difference between transparent and white opaque is dependent upon sizes of spherical crystals. However, it is important that crystallisation does not further proceed when these containers are heat-treated and utilised, and they thus are not whitened to be opaque and brittle.
In order to realise the above inexpensive and heat-resistant moulds, recovered PET bottles and recovered PET sheets available at prices half of those of commodity resins may be used as base materials resulting in, however, moulds of low molecular weights and of poor quality. Accordingly, larger molecular weights, higher melt viscosity, and high speeds of crystallisation are required. However, molecular weights of conventional PET become larger by condensation following solid phase polymerisation, but “linear structures” only are generated. Thus, higher melt viscosity and high speeds of crystallisation cannot be achieved. Accordingly, neck-in is large in moulding sheets, and drawn-down also is large at high temperatures in pressure-moulding or vacuum-pressure-moulding of sheets. Furthermore, efficient production cannot be achieved due to slow crystallisation rates of several minutes resulting in elongation of moulding process cycle and in difficulty to realise inexpensive heat-resistant moulds. On the other hand, A-PET without heat-setting generally has crystallisation temperatures (Tc according to slowly, i.e. 10° C./minute, increasing temperature of DSC method) of around 120-130° C., resulting in 5-6% of small crystallisation degree not to give heat-resistance.
An object of the present invention is to invent sheets and boards capable of being crystallised rapidly, and to provide a manufacturing method of inexpensive transparent or white opaque heat-resistant mould articles made of PET polyesters suitable for transparent blister packs heat-resistant against 80° C. or higher, transparent windows of automatic vendors, transparent containers stable even against heating and filling, transparent containers heat-resistant against 110-140° C. by microwave ovens, and white opaque containers heat-resistant against 180-220° C. by oven ranges, by control of transparency and opacity by management of sizes of the spherical crystals.